1. Field
Advancements in concentrated solar thermal power (CST), photovoltaic solar energy (PV), concentrated photovoltaic solar energy (CPV), and industrial use of concentrated solar thermal energy are needed to provide improvements in performance, efficiency, and utility of use.
2. Related Art
Unless expressly identified as being publicly or well known, mention herein of techniques and concepts, including for context, definitions, or comparison purposes, should not be construed as an admission that such techniques and concepts are previously publicly known or otherwise part of the prior art. All references cited herein (if any), including patents, patent applications, and publications, are hereby incorporated by reference in their entireties, whether specifically incorporated or not, for all purposes.
Concentrated solar power systems use mirrors, known as concentrators, to gather solar energy over a large space and aim and focus the energy at receivers that convert incoming solar energy to another form, such as heat or electricity. There are several advantages, in some usage scenarios, to concentrated systems over simpler systems that directly use incident solar energy. One advantage is that more concentrated solar energy is more efficiently transformed to heat or electricity than less concentrated solar energy. Thermal and photovoltaic solar receivers operate more efficiently at higher incident solar energy levels. Another advantage is that non-concentrated solar energy receivers are, in some usage scenarios, more expensive than mirror systems used to concentrate sunlight. Thus, by building a system with mirrors, total cost of gathering sunlight over a given area and converting the gathered sunlight to useful energy is reduced.
Concentrated solar energy collection systems, in some contexts, are divided into four types based on whether the solar energy is concentrated into a line-focus receiver or a point-focus receiver and whether the concentrators are single monolithic reflectors or multiple reflectors arranged as a Fresnel reflector to approximate a monolithic reflector.
A line-focus receiver is a receiver with a target that is a relatively long straight line, like a pipe. A line-focus concentrator is a reflector (made up of a single smooth reflective surface, multiple fixed facets, or multiple movable Fresnel facets) that receives sunlight over a two dimensional space and concentrates the sunlight into a significantly smaller focal point in one dimension (width) while reflecting the sunlight without concentration in the other dimension (length) thus creating a focal line. A line-focus concentrator with a line-focus receiver at its focal line is a basic trough system. The concentrator is optionally rotated in one dimension around its focal line to track daily or seasonal (apparent) movement of the sun to improve total energy capture and conversion.
A point-focus receiver is a receiver target that is essentially a point, but in various approaches is a panel, window, spot, ball, or other target shape, generally more equal in width and length than a line-focus receiver. A point-focus concentrator is a reflector (made up of a single smooth reflective surface, multiple fixed facets, or multiple movable Fresnel facets) that receives sunlight over a two-dimensional space and concentrates the sunlight into a significantly smaller focal point in two dimensions (width and length). A monolithic point-focus concentrator with a point-focus receiver at its focal point is a basic dish concentrated solar system. The monolithic concentrator is optionally rotated in two dimensions to rotate its focal axis around its focal point to track daily and seasonal movement of the sun to improve total energy capture and conversion.
A parabolic trough system is a line concentrating system using a monolithic reflector shaped like a large half pipe having a shape defined by the equation y2=4fx where f is the focal length of the trough. The reflector has a 1-dimensional curvature to focus sunlight onto a line-focus receiver or approximates such curvature through multiple facets fixed relative to each other.
A concentrating Fresnel reflector is a line concentrating system similar to the parabolic trough replacing the trough with a series of mirrors, each the length of a receiver, that are flat or alternatively slightly curved in width. Each mirror is individually rotated about its long axis to aim incident sunlight onto the line-focus receiver.
A parabolic dish system is a point concentrating system using a monolithic reflector shaped like a bowl. The reflector has a 2-dimensional curvature to focus sunlight onto a point-focus receiver or approximates such curvature through multiple flat or alternatively curved facets fixed relative to each other.
A solar power tower is a point concentrating system similar to the parabolic dish, replacing the dish with a 2-dimensional array of mirrors that are flat or alternatively curved. Each mirror (heliostat) is individually rotated in two dimensions to aim incident sunlight onto a point-focus receiver. The individual mirrors and an associated control system are parts of a point-focus concentrator with a focal axis that rotates around its focal point.
In solar thermal systems, the receiver is a light to heat transducer. The receiver absorbs solar energy, transforming it to heat and transmitting the heat to a thermal transport medium such as water, steam, oil, or molten salt. The receiver converts solar energy to heat and minimizes and/or reduces heat loss due to thermal radiation. In concentrated photovoltaic systems, the receiver is a photovoltaic surface that directly generates electricity from sunlight. In some solar thermal systems, CPV and CST are combined in a single system where a thermal energy system generates thermal energy and acts as a heat sink for photovoltaic cells that operate more efficiently when cooled. Other receivers, such as a stirling engine, that use solar energy to generate heat and then locally convert the heat to electricity through mechanical motion and an electric generator, are also deployed as a receiver, in some approaches.
In some concentrated solar systems, such as some systems with high concentration ratios, overall system is cost dominated by various elements such as the concentration system (such as a mirror or lens), a support system for the concentrators, and motors and mechanisms that enable tracking movement of the sun. The elements dominate the costs because the elements are enabled to withstand wind and weather. In some usage scenarios, solar energy systems are enabled to withstand various environmental dangers such as wind, rain, snow, ice, hail, dew, rodents, birds and other animals, dust, sand, moss, and other living organisms. Reflectivity of a concentrator is sensitive to damage, tarnishing, and dirt buildup since only directly reflected sunlight, not scattered sunlight, is effectively focused.
Glass mirrors are used in some concentrated systems, because of an ability to maintain good optical properties over long design lives (e.g. 30 years) of concentrated solar systems. Glass is relatively fragile and vulnerable to hail and other forms of damage unless it is suitably thick, e.g. 4-5 mm for relatively larger mirrors. In a 400 square foot concentrating dish the thickness results in a weight of close to 1000 lbs or about nine kg per square meter of concentrator area. The mirror is formed in a precise curve, in one dimension for a trough, in two dimensions for a dish, to focus sunlight.
In some concentrated systems, mirror surfaces cease to focus as intended if warped. Thus, the reflector is supported and held in shape by a metal superstructure that is shaped to the curved glass. The superstructure supports and protects the mirror from environmental conditions such as winds of 75 mph or more. The protection from winds adds an additional 10,000 lbs of load beyond the 1000 lb weight of the mirror, resulting in complex construction requiring roughly 20 kg of structural steel for every square meter of mirror area in a dish system.
In some concentrated systems, concentrator tracking motors move the 30 kg per square meter weight of the concentrator, and also overcome force of wind that exceeds an additional 300 kg per square meter. The motors are exposed to environmental elements (such as, dirt, dust, moisture, etc).
In some concentrated systems, troughs are spaced relatively far apart on (e.g. level) ground to avoid shading each other. Avoiding shading is important because the trough mirror is relatively expensive and so having any mirror shaded (and unproductive) is costly. Few approaches exceed ground coverage of 33% since that spacing avoids shading in winter (for an east/west orientation) or early/late in the day (for a north/south orientation). Some east/west orientations (e.g. with 33% ground coverage), have essentially no shading of any trough surface by another at any time during the day or year, capture almost all incident light within a trough array boundary during the winter (when the troughs are held vertical), but capture only about ⅓ of incident light within the the trough array boundary during the summer (when the troughs are held horizontal).
Troughs are placeable with length running north/south or east/west. If placed running north/south, then they are rotated during the day to track the daily movement of the sun and keep incident light focused on the receiver. In the morning, the trough is aimed to the east at the rising sun, at noon it is aimed up at the noonday sun, and in the evening it is aimed to the west at the setting sun. North/south troughs do not track seasonal variation in sun position. Instead, as the sun moves lower in the sky (toward the horizon of the equator) during the winter, light strikes the trough and reflects up the trough (away from the equator) to the receiver. In some instances, if troughs are oriented east/west, then they are rotated as the seasons progress to aim at the sun. During the summer the trough is held somewhat horizontal, aimed more or less straight up at the summer sun. During the winter the trough is held somewhat vertical, aimed toward the sun that is lower in the sky in the direction of the equator. In some instances, an east/west trough does not track the daily motion of the sun. Instead, when light comes from the east in the morning, it reflects off the trough and travels further west until it hits the receiver. Similarly, in the evening, the light hits the trough and travels further east down the trough until it hits the receiver.